The present invention relates to apparatus and methods useful in connection with performing electrical cardioversion/defibrillation and optional pacing of the heart.
Defibrillation/cardioversion is a technique employed to counter arrhythmic heart conditions including some tachycardias in the atria and/or ventricles. Typically, electrodes are employed to stimulate the heart with electrical impulses or shocks, of a magnitude substantially greater than pulses used in cardiac pacing.
Defibrillation/cardioversion systems include body implantable electrodes that are connected to a hermetically sealed container housing the electronics, battery supply and capacitors. The entire system is referred to as implantable cardioverter/defibrillators (ICDs). The electrodes used in ICDs can be in the form of patches applied directly to epicardial tissue, or, more commonly, are on the distal regions of small cylindrical insulated catheters that typically enter the subclavian venous system, pass through the superior vena cava and, into one or more endocardial areas of the heart. Such electrode systems are called intravascular or transvenous electrodes. U.S. Pat. Nos. 4,603,705, 4,693,253, 4,944,300, 5,105,810, the disclosures of which are all incorporated herein by reference, disclose intravascular or transvenous electrodes, employed either alone, in combination with other intravascular or transvenous electrodes, or in combination with an epicardial patch or subcutaneous electrodes. Compliant epicardial defibrillator electrodes are disclosed in U.S. Pat. Nos. 4,567,900 and 5,618,287, the disclosures of which are incorporated herein by reference. A sensing epicardial electrode configuration is disclosed in U.S. Pat No. 5,476,503, the disclosure of which is incorporated herein by reference.
In addition to epicardial and transvenous electrodes, subcutaneous electrode systems have also been developed. For example, U.S. Pat. Nos. 5,342,407 and 5,603,732, the disclosures of which are incorporated herein by reference, teach the use of a pulse monitor/generator surgically implanted into the abdomen and subcutaneous electrodes implanted in the thorax. This system is far more complicated to use than current ICD systems using transvenous lead systems together with an active can electrode and therefore it has no practical use. It has in fact never been used because of the surgical difficulty of applying such a device (3 incisions), the impractical abdominal location of the generator and the electrically poor sensing and defibrillation aspects of such a system.
Recent efforts to improve the efficiency of ICDs have led manufacturers to produce ICDs which are small enough to be implanted in the pectoral region. In addition, advances in circuit design have enabled the housing of the ICD to form a subcutaneous electrode. Some examples of ICDs in which the housing of the ICD serves as an optional additional electrode are described in U.S. Pat. Nos. 5,133,353, 5,261,400, 5,620,477, and 5,658,321 the disclosures of which are incorporated herein by reference.
ICDs are now an established therapy for the management of life threatening cardiac rhythm disorders, primarily ventricular fibrillation (V-Fib). ICDs are very effective at treating V-Fib, but are therapies that still require significant surgery.
As ICD therapy becomes more prophylactic in nature and used in progressively less ill individuals, especially children at risk of cardiac arrest, the requirement of ICD therapy to use intravenous catheters and transvenous leads is an impediment to very long term management as most individuals will begin to develop complications related to lead system malfunction sometime in the 5-10 year time frame, often earlier. In addition, chronic transvenous lead systems, their reimplantation and removals, can damage major cardiovascular venous systems and the tricuspid valve, as well as result in life threatening perforations of the great vessels and heart. Consequently, use of transvenous lead systems, despite their many advantages, are not without their chronic patient management limitations in those with life expectancies of  greater than 5 years. The problem of lead complications is even greater in children where body growth can substantially alter transvenous lead function and lead to additional cardiovascular problems and revisions. Moreover, transvenous ICD systems also increase cost and require specialized interventional rooms and equipment as well as special skill for insertion. These systems are typically implanted by cardiac electrophysiologists who have had a great deal of extra training.
In addition to the background related to ICD therapy, the present invention requires a brief understanding of a related therapy, the automatic external defibrillator (AED). AEDs employ the use of cutaneous patch electrodes, rather than implantable lead systems, to effect defibrillation under the direction of a bystander user who treats the patient suffering from V-Fib with a portable device containing the necessary electronics and power supply that allows defibrillation. AEDs can be nearly as effective as an ICD for defibrillation if applied to the victim of ventricular fibrillation promptly, i.e., within 2 to 3 minutes of the onset of the ventricular fibrillation.
AED therapy has great appeal as a tool for diminishing the risk of death in public venues such as in air flight. However, an AED must be used by another individual, not the person suffering from the potential fatal rhythm. It is more of a public health tool than a patient-specific tool like an ICD. Because  greater than 75% of cardiac arrests occur in the home, and over half occur in the bedroom, patients at risk of cardiac arrest are often alone or asleep and can not be helped in time with an AED. Moreover, its success depends to a reasonable degree on an acceptable level of skill and calm by the bystander user.
What is needed therefore, especially for children and for prophylactic long term use for those at risk of cardiac arrest, is a combination of the two forms of therapy which would provide prompt and near-certain defibrillation, like an ICD, but without the long-term adverse sequelae of a transvenous lead system while simultaneously using most of the simpler and lower cost technology of an AED. What is also needed is a cardioverter/defibrillator that is of simple design and can be comfortably implanted in a patient for many years.
One feature desirable in such a cardioverter/defibrillator is one which permits a physician in the field to test the energy level of an implanted ICD to ensure that the ICD will deliver an effective defibrillating pulse. Once the physician has determined that a sufficient amount of energy will be delivered to achieve defibrillation, he or she can complete the implant procedure.
In order to test the energy level of the ICD, it is necessary to induce fibrillation and then observe whether the ICD counteracts it. To induce fibrillation, a shock is delivered to a patient which coincides with ventricular repolarization. This repolarization coincides with the T-wave portion of the patient""s cardiac waveform shown in FIG. 1. This waveform includes the P, Q, R, and S peaks, followed by the T-wave. The Q, R, and S pulses may collectively be referred to as the xe2x80x9cQRS Complexxe2x80x9d 12, or the xe2x80x9cR-wave.xe2x80x9d Thus, delivering a small energy shock in coincidence with occurrence of the T-wave will induce fibrillation.
In pacing systems, delivering a shock on the T-wave is relatively straight-forward because, in such systems, pacing is done at a rate which exceeds the patient""s intrinsic heart rhythm, and the ventricle is stimulated with a programmed constant coupling interval. In contrast, when the patient""s heart is free-running, the distance between the T-wave and the QRS complex can vary.
Thus, with pacing control, one knows precisely when the QRS complex and the T-wave will occur. As shown in FIG. 2, a first pacing QRS signal S1 is applied and, after an initial response or xe2x80x9ccouplingxe2x80x9d interval I, a T-wave occurs. Then, after the coupling interval, another pacing QRS signal S2 is applied and, after a second coupling interval I, another T-wave occurs. It is then possible to determine exactly when the T-wave will occur from examining the response intervals. Accordingly, with pacing, one simply defines how many pulses S1, S2, etc., to deliver and then delivers a shock at the desired position on the T-wave. Typically, the shock is anywhere from a half (0.5) a joule to 350 joules and may be delivered as a monophasic or biphasic pulse or other signal shape.
In pacing systems, one has the luxury of entraining the heart because an electrical lead is positioned in the heart. The present invention, however, is intended to be useful in systems where a lead is not located in the heart and where pacing is not available. Accordingly, the subject invention facilitates employing the patient""s own intrinsic cardiac signal to control delivery of a shock on the T-wave.
Since the patient""s intrinsic heart rate varies, the timing of the T-wave varies. Accordingly, according to the invention, the occurrence of the T-wave may be automatically detected and a shock delivered in response to the detection. The invention further contemplates manual and pacing embodiments where the timing of the shock is manually set to occur at a selected point in time following automatic detection of the QRS complex or R-wave.